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    <td width="885"> <font face="Verdana, Arial, Helvetica, sans-serif"><b><font size="6">The 
      Tracing Facility</font></b></font></td>
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<p>If you ever had the need to debug a macro expansion you had to discover, that 
  your tools provide only little or no support for this task. For this reason 
  the <i>Wave</i> library has a tracing facility, which allows to get selectively 
  some information about the expansion of a certain macro or several macros. </p>
<p>The tracing of macro expansions generates a possibly huge amount of information, 
  so it is recommended, that you explicitly enable/disable the tracing for the
  macro in question only. This may be done with the help of a special, <tt>Wave</tt> 
  specific #pragma:</p>
<pre><span class="preprocessor">    #pragma</span> wave trace(enable)    <span class="comment">// enable the tracing</span>
    <span class="comment">// the macro expansions here will be traced</span>
    <span class="comment">// ...</span>
<span class="preprocessor">    #pragma</span> wave trace(disable)   <span class="comment">// disable the tracing</span></pre>
<p>In C99 mode or when specifying the <tt>--variadics</tt> command line option 
  you may additionally use the <tt>operator&nbsp;_Pragma()</tt> variant to enable/disable 
  the tracing output:</p>
<pre><span class="preprocessor">    #define</span> CONCAT(x, y) \
        <span class="preprocessor">_Pragma</span>(<span class="string">&quot;wave trace(enable)&quot;</span>) \
        x \
        <span class="preprocessor">_Pragma</span>(<span class="string">&quot;wave trace(disable)&quot;</span>) \
        <span class="keyword">##</span> y</pre>
<p>This way you have the possibility to enable the tracing during the expansion 
  of a part of a macro only. In the sample shown there is traced the expansion 
  of the macro argument <tt>'x'</tt> only. Note, that the <tt>operator _Pragma()</tt> 
  directives expand to nothing inside the macro expansion result.</p>
<p>To see, what the <tt>Wave</tt> driver generates while expanding a simple macro, 
  let's have a look at the tracing output for the following example:</p>
<pre ><span class="comment">    // test.cpp</span>
<span class="preprocessor">    #define</span> X(x)          x<br><span class="preprocessor">    #define</span> Y()           2<br><span class="preprocessor">    #define</span> CONCAT_(x, y) x <span class="keyword">##</span> y
<span class="preprocessor">    #define</span> CONCAT(x, y)  CONCAT_(x, y)
<span class="preprocessor">    #pragma</span> wave trace(enable)
<span class="comment">    // this macro expansion is to be traced</span>
    CONCAT(X(1), Y())     <span class="comment">// should expand to 12</span>
<span class="preprocessor">    #pragma</span> wave trace(disable)</pre>
<p>When preprocessed with <tt>'wave -t test.trace test.cpp'</tt> the <tt>Wave</tt> 
  driver generates a file <tt>test.trace</tt>, which contains (without the line 
  numbers in front of the lines):</p>
<pre>  1: test.cpp:8:1: CONCAT(X(1), Y())
  2:   test.cpp:5:9: see macro definition: CONCAT(x, y)
  3:   invoked with
  4:   [
  5:     x = X(1)
  6:     y = Y()
  7:   ]
  8:   [
  9:     test.cpp:2:9: see macro definition: X(x)
 10:     invoked with
 11:     [
 12:       x = 1
 13:     ]
 14:     [
 15:       1
 16:       rescanning
 17:       [
 18:         1
 19:       ]
 20:     ]
 21:     test.cpp:3:9: see macro definition: Y()
 22:     [
 23:       2
 24:       rescanning
 25:       [
 26:         2
 27:       ]
 28:     ]
 29:     CONCAT_(1, 2)
 30:     rescanning
 31:     [
 32:       test.cpp:4:9: see macro definition: CONCAT_(x, y)
 33:       invoked with
 34:       [
 35:         x = 1
 36:         y = 2
 37:       ]
 38:       [
 39:         12
 40:         rescanning
 41:         [
 42:           12
 43:         ]
 44:       ]
 45:       12
 46:     ]
 47:   ]
</pre>
<p>The generated trace output is very verbose, but allows to follow every step 
  of the actual macro expansion process. The first line in this tracing example 
  contains the reference to the position, from where the macro expansion was initiated. 
  Additionally the following information is contained for every single macro expansion:</p>
<ul>
  <li>The reference to the position (line and column numbers), where the macro to expand was defined first 
    (see lines 2, 9, 21 and 32).</li>
  <li>The real parameters supplied for this macro expansion (see lines 3, 10 and 
    33), this information is traced inside the <tt>invoked with</tt> block, where 
    the corresponding formal and actual parameters are listed.</li>
  <li>The expansion of the given arguments (if any and if these are defined as 
    macros). This repeats the full tracing information for the argument macro 
    expansion, only indented by one level. Note though, that the macro expansion
    of the actual arguments is traced, regardless of the fact, whether this argument 
    is really to be inserted into the replacement list after its expansion 
    or as it was initially supplied (see C++ Standard [16.3.1.1]: &quot;A parameter 
    in the replacement list, unless preceded by a <tt>#</tt> or <tt>##</tt> preprocessing 
    token or followed by a <tt>##</tt> preprocessing token, is replaced by the 
    corresponding argument after all macros contained therein have been expanded&quot; 
    <a href="references.html#iso_cpp">[1]</a>). </li>
  <li>The result of the argument substitution (see lines 15, 23, 29 and 39), i.e. 
    the substituted replacement list.</li>
  <li>The rescanning process, which again includes the full subsequent macro expansion 
    process of all found macros (see C++ Standard [16.3.4.1]: &quot;After all 
    parameters in the replacement list have been substituted, the resulting preprocessing 
    token sequence is rescanned with all subsequent preprocessing tokens of the 
    source file for more macro names to replace.&quot; <a href="references.html#iso_cpp">[1]</a>).</li>
  <li>The result of the actual macro expansion (this is the last line inside the 
    corresponding rescanning block - see lines 18, 26, 42 and 45).</li>
</ul>
<p>Every found macro to expand will add an additional indentation level inside 
  the trace output.</p>
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<p class="copyright">Copyright &copy; 2003-2011 Hartmut Kaiser<br>
  <br>
<font size="2">Distributed under the Boost Software License, Version 1.0. (See accompanying file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt) </font> </p>
<p class="copyright"><span class="updated">Last updated: 
  <!-- #BeginDate format:fcAm1m -->Tuesday, March 21, 2006  9:25<!-- #EndDate -->
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